Conventional cement is produced by superheating a mixture of limestone and other materials, such as clay, sand etc. at around 1450° C. The most commonly used cement is Portland cement, with a composition of tri-calcium silicate (C3S in cement chemist notation), di-calcium silicate (C2S), tri-calcium aluminate (C3A) and tetra-calcium aluminoferrite (C4AF). The composition of Portland cement is shown in Table 1A.
TABLE 1AComposition of Portland CementCaOSiO2Al2O3Fe2O362~67 wt %20~24 wt %4~7 wt %3~5 wt %
White ordinary Portland cement (WOPC) is similar to Ordinary Portland cement (OPC) except for the color. WOPC is snow white in color and can be used to produce different colors of concrete when mixed with colored pigments. The uses of white cement are limited compared to OPC because its manufacturing cost is much more than that of OPC. Therefore, WOPC has not been widely used as a substitute for OPC.
The greenish-gray to brown color of ordinary Portland cement is derived from a number of transitional elements like chromium, manganese, iron, copper, vanadium, nickel and titanium. These elements find very little space in the composition of white cement. In particular, the amount of Cr2O3 is maintained below 0.003%, Mn2O3 is maintained below 0.03%, and Fe2O3 is maintained below 0.35% in the clinker. The other elements are usually not a significant problem. Portland cement is generally made from limestone, clay, shale, iron ore, bauxite and sand. This usually contains substantial amounts of Cr, Mn and Fe. Limestone used in cement manufacture contains 0.3-1% Fe2O3, whereas levels below 0.1% are sought in limestone for white cement manufacture.
Conventionally, WOPC is produced by superheating the raw mix to higher peak in temperature of (1450-1500° C.) than that of the OPC where the peak is (1400-1450° C.). The fuel consumption to produce WOPC is 20-50% more than that of OPC and also results in lower kiln output ranging from 20-50% less for a given sized kiln. The composition of white Portland cement is shown in Table 1B.
TABLE 1BComposition Of White Portland CementCaOSiO2Al2O3Fe2O366.3 wt %22.5 wt %4.5 wt %0.3 wt %
The hydration rate of cement after mixing with water depends on the surface area of the cement. The final product has to be ground to reduce the size of cement particles. Commercially available cements have particle sizes ranging from a few microns to 60 microns with a surface area from 0.3 to 1.2 m2/g. It usually takes 7-14 days for the setting of micron-sized cement particles. Nano size cement particles will react with water in a very fast rate, which will offer applications in building renovations, sealing and as an accelerating additive to presently used cements.
Conventional cement manufacturing is very energy intensive, consuming about 3 to 6 GJ of energy per ton of ordinary cement produced. The manufacture of white cement consumes about 5 to 8 GJ of energy per ton of white cement produced. In the year 2007, 2600 million tons of cement was produced worldwide, which accounts for 8.3 percent of global industrial energy use. The chemical reactions involved in heating limestone and the burning of the fuel gives off CO2, which contributes to 8% of global CO2 emissions. It remains a challenge to reduce the energy cost, as well as the CO2 emissions from the cement manufacturing process for a sustainable point of view. Many modifications have been done to reduce the energy usage and CO2 emissions, but these modifications did not significantly contribute to the cause.
Combustion synthesis (CS), originally called self-propagating high-temperature synthesis (SHS), is the synthesis of materials or compounds using self-sustaining highly exothermic redox chemical reactions (combustion). It has been widely applied for the synthesis of metal oxides. Cement prepared by combustion synthesis has the following advantages over conventional manufacturing processes:                the cement particles prepared by CS are in nano sized range, with very fast hydration rate compared with conventional Portland cement;        the equipment used in CS is relatively simple which lowers the capital cost;        the CS process has low energy requirements. The energy released from the exothermic reaction, which is usually ignited at a relative low temperature, can rapidly heat the raw materials to a high temperature and sustain long enough for the synthesis to complete. Therefore, no constant external heat supply is necessary. The process can reduce the energy cost by 40-50% and lower the CO2 emissions by at least 20%; and        the CS process can be completed in seconds due to the fast reaction rate.        